CMAS-Resistant Thermal Barrier Coatings

ABSTRACT

A coating including a CMAS-resistant layer with a rare earth oxide. The CMAS-resistant layer is essentially free of zirconia and hafnia, and may further include at least one of alumina, silica, and combinations thereof.

TECHNICAL FIELD

The present disclosure generally relates to thermal barrier coatings forhigh-temperature mechanical systems, such as gas turbine engines, andmore particularly to thermal barrier coatings including rare earthoxides.

BACKGROUND

The components of high-temperature mechanical systems, such as, forexample, gas-turbine engines, must operate in severe environments. Forexample, the high-pressure turbine blades and vanes exposed to hot gasesin commercial aeronautical engines typically experience metal surfacetemperatures of about 1000° C., with short-term peaks as high as 1100°C. Typical components of high-temperature mechanical systems include aNi or Co-based superalloy substrate. In an attempt to reduce thetemperatures experienced by the substrate, the substrate can be coatedwith a thermal barrier coating (TBC). The thermal barrier coating mayinclude a thermally insulative ceramic topcoat and is bonded to thesubstrate by an underlying metallic bond coat. The TBC, usually appliedeither by air plasma spraying or electron beam physical vapordeposition, is most often a layer of yttria-stabilized zirconia (YSZ)with a thickness of about 100-500 μm. The properties of YSZ include lowthermal conductivity, high oxygen permeability, and a relatively highcoefficient of thermal expansion. The YSZ TBC is also typically made“strain tolerant” and the thermal conductivity further lowered bydepositing a structure that contains numerous pores and/or pathways.

Economic and environmental concerns, i.e., the desire for improvedefficiency and reduced emissions, continue to drive the development ofadvanced gas turbine engines with higher inlet temperatures. As theturbine inlet temperature continues to increase, there is a demand for aTBC with lower thermal conductivity and higher temperature stability tominimize the increase in, maintain, or even lower the temperaturesexperienced by substrate.

SUMMARY

In general, the invention is directed to a TBC or EBC topcoat havingenhanced CMAS-resistance compared to conventional YSZ topcoats. CMAS isa calcia-magnesia-alumina-silicate deposit resulting from the ingestionof siliceous minerals (dust, sand, volcanic ashes, runway debris, andthe like) with the intake of air in gas turbine engines.

In one aspect, the disclosure is directed to a coating with aCMAS-resistant layer including a rare earth oxide, wherein theCMAS-resistant layer is essentially free of zirconia and hafnia.

In another aspect the disclosure is directed to a coating with aCMAS-resistant layer including a rare earth oxide and a second layer.The second layer includes a compound selected from a MCrAlY alloy,wherein M is selected from Ni, Co, and NiCo; a β-NiAl alloy; aγ-Ni+γ′-Ni₃Al alloy; rare earth oxide-stabilized zirconia, rare earthoxide-stabilized hafnia, mullite, silicon, barium strontiumaluminosilicate, calcium aluminosilicate, cordierite, lithiumaluminosilicate, rare earth silicates, and combinations thereof. TheCMAS-resistant layer is adjacent the second layer.

In yet another aspect, the disclosure is directed to an article with asubstrate and a CMAS-resistant layer including a rare earth oxide,wherein the first layer is essentially free of zirconia and hafnia.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are cross-sectional diagrams of a substrate coated witha bond coat and a thermal or environmental barrier coating including aCMAS-resistant layer.

FIG. 2 is a cross-sectional diagram of an alternative embodiment of asubstrate coated with a bond coat and a thermal or environmental barriercoating including a transition layer and a CMAS-resistant layer.

FIG. 3 is a cross-sectional diagram of a substrate coated with a bondcoat and a thermal or environmental barrier coating including atransition layer and a CMAS-resistant layer.

FIGS. 4A, 4B, and 4C are cross-sectional diagrams illustratingtransitional layers between a thermal barrier coating and aCMAS-resistant layer.

FIG. 5 is a cross-sectional diagram of a substrate coated with a bondcoat and a CMAS-resistant layer.

FIGS. 6A and 6B are cross-sectional photographs of an ytterbium silicate(Yb₂Si₂O₇) layer in contact with a CMAS layer and a reaction layerformed at the interface.

FIGS. 7A and 7B are cross-sectional photographs of an ytterbium silicate(Yb₂Si₂O₇) layer in contact with a CMAS layer and a reaction layerformed at the interface.

FIGS. 8A and 8B are cross-sectional photographs of an ytterbium silicate(Yb₂SiO₅) layer in contact with a CMAS layer and a reaction layer formedat the interface.

FIG. 9 is a cross-sectional photograph of an ytterbium silicate(Yb₂SiO₅) layer in contact with two CMAS layers and reaction layersformed at the interfaces of the ytterbium silicate layer and the CMASlayers.

FIG. 10 is a cross-sectional photograph of an ytterbium silicate(Yb₂Si₂O₇) layer bonded to a CMC substrate by a Si bond coat layer.

DETAILED DESCRIPTION

In general, the disclosure is directed to thermal barrier coating (TBC)compositions that possess increased CMAS(calcia-magnesia-alumina-silicate) degradation resistance compared toconventional yttria-stabilized zirconia (YSZ) TBCs, and articles coatedwith such TBCs. More specifically, the disclosure is directed to athermal barrier coating including a CMAS-resistant layer that reactswith CMAS leading to increased CMAS degradation resistance compared toconventional YSZ TBCs.

As turbine inlet temperatures continue to increase, new thermal barriercoatings are required with better high temperature performance. Asdescribed briefly above, TBCs are typically deposited as a porousstructure, which increases the stress tolerance and reduces the thermalconductivity of the TBC. However, this porous structure is susceptibleto damage. Higher turbine inlet temperatures may lead to damage of theTBC when CMAS, a calcia-magnesia-alumina-silicate deposit, is formedfrom the ingestion of siliceous minerals (dust, sand, volcanic ashes,runway debris, and the like) with the intake of air in gas turbineengines. Typical CMAS deposits have a melting temperature of about 1200°C. to about 1250° C. (about 2200° F. to about 2300° F.). As advancedengines run at TBC surface temperatures above the CMAS meltingtemperature, the molten CMAS may infiltrate the pores of the TBC. Whenthe component is cooled below the CMAS melting temperature, the CMASsolidifies, which exerts a strain on the TBC and may reduce its usefullife. The filling of the pores of the TBC with molten CMAS may alsoincrease the thermal conductivity of the TBC, which is detrimental tothe TBC performance and causes higher thermal stress on the componentsubstrate.

Additionally, the molten CMAS may dissolve the YSZ TBC. The YSZ TBCdissolves preferentially along grain boundaries, and depending on themelt chemistry, zirconia with lower yttria content may precipitate outof the molten solution, thus decreasing the effectiveness of the TBC.

FIGS. 1A and 1B show cross-sectional views of an exemplary article 10used in a high-temperature mechanical system. The article 10 includes acoating 14 applied to a substrate 12. The coating 14 includes a bondcoat 11 applied to substrate 12, a thermal barrier coating (TBC) orenvironmental barrier coating (EBC) 13 applied to the bond coat 11, anda CMAS-resistant layer 15 applied to the TBC/EBC 13. The choice ofincluding a TBC or an EBC may depend on the substrate, as will bedescribed in more detail below.

The substrate 12 may be a component of a high temperature mechanicalsystem, such as, for example, a gas turbine engine or the like. Typicalsuperalloy substrates 12 include alloys based on Ni, Co, Ni/Fe, and thelike. The superalloy substrate 12 may include other additive elements toalter its mechanical properties, such as toughness, hardness,temperature stability, corrosion resistance, oxidation resistance, andthe like, as is well known in the art. Any useful superalloy substrate12 may be utilized, including, for example, those available fromMartin-Marietta Corp., Bethesda, Md., under the trade designationMAR-M247; those available from Cannon-Muskegon Corp., Muskegon, Mich.,under the trade designations CMSX-4 and CMSX-10; and the like.

The substrate 12 may also include a ceramic matrix composite (CMC). TheCMC may include any useful ceramic matrix material, including, forexample, silicon carbide, silicon nitride, alumina, silica, and thelike. The CMC may further include any desired filler material, and thefiller material may include a continuous reinforcement or adiscontinuous reinforcement. For example, the filler material mayinclude discontinuous whiskers, platelets, or particulates. As anotherexample, the filler material may include a continuous monofilament ormultifilament weave.

The filler composition, shape, size, and the like may be selected toprovide the desired properties to the CMC. For example, in someembodiments, the filler material may be chosen to increase the toughnessof a brittle ceramic matrix. In other embodiments, the filler may bechosen to provide a desired property to the CMC, such as thermalconductivity, electrical conductivity, thermal expansion, hardness, orthe like.

In some embodiments, the filler composition may be the same as theceramic matrix material. For example, a silicon carbide matrix maysurround silicon carbide whiskers. In other embodiments, the fillermaterial may include a different composition than the ceramic matrix,such as mullite fibers in an alumina matrix, or the like. One preferredCMC includes silicon carbide continuous fibers embedded in a siliconcarbide matrix.

The article 10 may include a bond coat 11 adjacent to or overlyingsubstrate 12. The bond coat 11 may improve adhesion between the TBC/EBC13 and the substrate 12. The bond coat 11 may include any useful alloy,such as a conventional MCrAlY alloy (where M is Ni, Co, or NiCo), aβ-NiAl nickel aluminide alloy (either unmodified or modified by Pt, Cr,Hf, Zr, Y, Si, and combinations thereof), a γ-Ni+γ′-Ni₃Al nickelaluminide alloy (either unmodified or modified by Pt, Cr, Hf, Zr, Y, Si,and combination thereof), or the like.

The bond coat 11 may also include ceramics or other materials that arecompatible with a CMC substrate 12. For example, the bond coat 11 mayinclude mullite, silicon, or the like. The bond coat 11 may furtherinclude other elements, such as silicates of rare earth elementsincluding lutetium, ytterbium, erbium, dysprosium, gadolinium, europium,samarium, neodymium, cerium, lanthanum, scandium, yttrium, or the like.Some preferred bond coat 11 compositions for overlying a CMC substrate12 include silicon, mullite, yttrium silicates and ytterbium silicates.

The bond coat 11 may be selected based on a number of considerations,including the chemical composition and phase constitution of the TBC/EBC13 and the substrate 12. For example, when the substrate 12 includes asuperalloy with γ-Ni+γ′-Ni₃Al phase constitution, the bond coat 11preferably includes a γ-Ni+γ′-Ni₃Al phase constitution to better matchthe coefficient of thermal expansion of the superalloy substrate 12, andtherefore increase the mechanical stability (adhesion) of the bond coat11 to the substrate 12. Alternatively, when the substrate 12 includes aCMC, the bond coat 11 is preferably silicon or a ceramic, such as, forexample, mullite.

In some embodiments, a bond coat 11 including a single layer may notfulfill all the functions desired of a bond coat 11. Thus, in somecases, the bond coat 11 may include multiple layers. For example, insome embodiments where the substrate 12 is a CMC comprising siliconcarbide, a bond coat including a layer of silicon followed by a layer ofmullite (aluminum silicate, Al₆Si₂O₁₃), a rare earth silicate, or amullite/rare earth silicate dual layer is deposited on the CMC substrate12. A bond coat 11 comprising multiple layers may be desirable on a CMCsubstrate 12 to accomplish the desired functions of the bond coat 11,such as, for example, adhesion of the substrate 12 to the TBC/EBC 13,chemical compatibility of the bond coat 11 with each of the substrate 12and the TBC/EBC 13, a desirable CTE match between adjacent layers, andthe like.

In yet other embodiments, the article 10 may not include a bond coat 11.For example, in some embodiments, the TBC/EBC 13 may be applied directlyto the substrate 12. A bond coat 11 may not be required or desired whenthe TBC/EBC 13 and the substrate 12 are chemically and/or mechanicallycompatible. For example, in embodiments where the TBC/EBC 13 andsubstrate 12 adhere sufficiently strongly to each other, a bond coat 11may not be necessary. Additionally, in embodiments where thecoefficients of thermal expansion of the substrate 12 and TBC/EBC 13 aresufficiently similar, a bond coat 11 may not be necessary. In this way,TBC/EBC 13 may be either adjacent to or overlie bond coat 11 or beadjacent to or overlie substrate 12.

TBC/EBC 13 may be selected to provide a desired type of protection tosubstrate 12. For example, when a substrate 12 including a superalloy isutilized, a thermal barrier coating may be desired to providetemperature resistance to substrate 12. A TBC, then, may provide thermalinsulation to substrate 12 to lower the temperature experienced bysubstrate 12. On the other hand, when a substrate 12 including a CMC isutilized, an EBC or an EBC/TBC bilayer or multilayer coating may bedesired to provide resistance to oxidation, water vapor attack, or thelike.

A TBC may include any useful insulative layer. Common TBCs includeceramic layers comprising zirconia or hafnia. The zirconia or hafnia TBCmay include other elements or compounds to modify a desiredcharacteristic of the TBC, such as, for example, phase stability,thermal conductivity, or the like. Exemplary additive elements orcompounds include, for example, rare earth oxides. The TBC may beapplied by any useful technique, including, for example, plasmaspraying, electron beam physical vapor deposition, chemical vapordeposition, and the like.

An EBC may include any useful layer which prevents environmental attackof the substrate. For example, the EBC may include materials that areresistant to oxidation or water vapor attack. Exemplary EBCs includemullite; glass ceramics such as barium strontium aluminosilicate(BaO—SrO—Al₂O₃-2SiO₂), calcium aluminosilicate (CaAl₂Si₂O₈), cordierite(magnesium aluminosilicate), and lithium aluminosilicate; and rare earthsilicates. The EBC may be applied by any useful technique, such asplasma spraying, electron beam physical vapor deposition, chemical vapordeposition and the like.

Regardless of whether coating 14 includes an EBC or a TBC, aCMAS-resistant layer 15 may be provided adjacent to or overlying TBC/EBC13 to protect the TBC/EBC 13 from infiltration of CMAS into the pores ofthe TBC/EBC 13. The CMAS-resistant layer 15 may react with any CMASpresent on the coating 14 and form a reaction layer 16, as shown in FIG.1B. The CMAS-resistant layer 15 and reaction layer 16 may form a barrierto reduce or prevent the infiltration of CMAS into the pores of theporous TBC/EBC 13.

In some embodiments, the CMAS-resistant layer 15 may be a distinctlayer, separate from TBC/EBC 13, as shown in FIGS. 1A and 1B. TheCMAS-resistant layer 15 may be applied to the TBC/EBC 13 using anyuseful method including, for example, plasma spraying, electron beamvapor deposition, chemical vapor deposition and the like.

The CMAS-resistant layer 15 may include any element that reacts withCMAS to form a solid or a highly-viscous reaction product (i.e., areaction product that is a solid or highly viscous at the temperaturesexperienced by the article 10). The reaction product may have a meltingtemperature significantly higher than CMAS (e.g., higher than about1200-1250° C.). A solid or highly viscous reaction product is desiredbecause the CMAS-resistant layer 15 is consumed as it reacts with CMASto form reaction layer 16. If, for example, the reaction product ofCMAS-resistant layer 15 and CMAS was a relatively low viscosity liquid,the low viscosity liquid would infiltrate the porous EBC/TBC 13 once theCMAS-resistant layer 15 is consumed by the reaction, which is the veryoccurrence the CMAS-resistant layer 15 is designed to prevent.

If the reaction product is a solid or highly viscous, however, areaction layer 16 will form on the surface of CMAS-resistant layer 15,which will lower the reaction rate of the CMAS with the CMAS-resistantlayer 15. That is, once a solid or highly viscous reaction layer 16forms on the surface of the CMAS-resistant layer 15, the reactionbetween the CMAS-resistant layer 15 and CMAS will slow, because anyfurther reaction will require the diffusion of CMAS through the reactionlayer 16 to encounter the CMAS-resistant layer 15, or diffusion of acomponent of the CMAS-resistant layer 15 through the reaction layer 16to encounter the CMAS. In either case, the diffusion of either CMAS orthe component of the CMAS-resistant layer 15 is expected to be thelimiting step in the reaction once a solid reaction layer 16 is formedon the surface of CMAS-resistant layer 15, because diffusion will be theslowest process.

The CMAS-resistant layer 15 includes at least one rare earth oxide.Useful rare earth oxides include oxides of rare earth elements,including, for example, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, Lu, and combinations thereof. In some embodiments, theCMAS-resistant layer 15 is essentially free of zirconia and hafnia. Thatis, in these embodiments, the coating includes at most trace amounts ofzirconia and hafnia, such as, for example, the amounts present incommercially-available rare earth oxides.

The CMAS-resistant layer 15 may also include alumina and/or silica inaddition to the at least one rare earth oxide. For example, theCMAS-resistant layer 15 may include alumina and at least one rare earthoxide, silica and at least one rare earth oxide, or alumina, silica, andat least one rare earth oxide. Alumina and/or silica may be added to theCMAS-resistant layer 15 to tailor one or more properties of theCMAS-resistant layer 15, such as, for example, the chemical reactivityof the layer 15 with CMAS, the viscosity of the reaction products, thecoefficient of thermal expansion, the chemical compatibility of thelayer 15 with the EBC/TBC 13, and the like.

Further, in some embodiments, the CMAS-resistant layer 15 may optionallyinclude other additive components, such as, for example, Ta₂O₅, HfSiO₄,alkali oxides, alkali earth oxides, or mixtures thereof. The additivecomponents may be added to the CMAS-resistant layer 15 to modify one ormore desired properties of the layer 15. For example, the additivecomponents may increase or decrease the reaction rate of theCMAS-resistant layer 15 with CMAS, may modify the viscosity of thereaction product from the reaction of CMAS and the CMAS-resistant layer15, may increase adhesion of the CMAS-resistant layer 15 to the TBC/EBC13, may increase or decrease the chemical stability of theCMAS-resistant layer 15, or the like.

The CMAS-resistant layer 15 may include from about 1 mol. % to about 100mol. % of the at least one rare earth oxide, ±1 mol. %. In someembodiments, the CMAS-resistant layer 15 may also include up to about 99mol. % of at least one of alumina, silica and combinations thereof, ±1mol. %, with a total of 100 mol. %.

In some preferred embodiments, the CMAS-resistant layer 15 may includeabout 10 mol. % to about 90 mol. % of at least one rare earth oxide, andabout 10 mol. % to about 90 mol. % of at least one of alumina, silica,and combinations thereof and, optionally, about 0.1 mol. % to about 50mol. % of the additive components. In other preferred embodiments, theCMAS-resistant layer 15 may include about 20 mol. % to about 80 mol. %of at least one rare earth oxide and about 20 mol. % to about 80 mol. %of at least one of alumina, silica, and combinations thereof and,optionally, about 1 mol. % to about 30 mol. % of the additivecomponents.

The thickness of the CMAS-resistant layer 15 may vary widely dependingon the conditions under which article 10 is to be used. For example, ifCMAS deposits are expected to be extensive, CMAS-resistant layer 15 maybe thicker. Additionally, if CMAS-resistant layer 15 is to replaceTBC/EBC 13, as will be described in further detail below, the thicknessof CMAS-resistant layer 15 may be determined by the thermal conditionsto which article 10 is exposed. The thickness depending on the intendedapplication may range from about 0.1 mils (1 mil=0.001 inch) to about 60mils, ±0.1 mil. In some embodiments, the thickness of CMAS-resistantlayer 15 may range from about 0.1 mils to about 30 mils. In otherembodiments, the thickness of CMAS-resistant layer 15 may range fromabout 0.5 mils to about 15 mils.

It may also be preferred that the coefficient of thermal expansion ofthe CMAS-resistant layer is similar to the coefficient of thermalexpansion of the TBC/EBC 13. Thus, the coefficient of thermal expansionof the component or components comprising the CMAS-resistant layer 15may be an important consideration when designing the CMAS-resistantlayer 15. Table 1 shows some exemplary rare earth silicates (e.g., arare earth oxide mixed with silica (SiO₂)) and their correspondingcoefficients of thermal expansion.

TABLE 1 Coefficients of Thermal Expansion for Various Rare Earth OxidesRare Earth Silicate Nd₂SiO₅ Gd₂SiO₅ Dy₂SiO₅ Yb₂SiO₅ Lu₂SiO₅ Yb₂Si₂O₇ CTE(10⁻⁶/° C.) 9.9 10.1 8.5 8 7.9 5.2

The exemplary rare earth silicates have coefficients of thermalexpansion that differ by as much as a factor of two (e.g.,Gd₂SiO₅(Gd₂O₃+SiO₂) and Yb₂Si₂O₇(Yb₂O₃+2SiO₂)). This permits a fairlywide range of tailoring of the coefficient of thermal expansion ofCMAS-resistant layer 15 to be similar to the TBC/EBC 13. For example, aTBC including yttria-stabilized zirconia has a coefficient of thermalexpansion of about 10×10⁻⁶/° C. Thus, either neodymium silicate(Nd₂SiO₅)-based or gadolinium silicate (Gd₂SiO₅)-based compositions maybe particularly desirable for including in a CMAS-resistant layer 15,along with any desired additive components. As a second example, a CMCsubstrate may have a coefficient of thermal expansion of about 4×10⁻⁶/°C. to about 5×10⁻⁶/° C. In embodiments where the CMAS-resistant layer 15is applied directly to a CMC substrate 12 or to a bond coat 11 attachedto a CMC substrate 12 (as will be described in further detail below),ytterbium silicate (Yb₂Si₂O₇)-based compositions may be a desirablechoice to include in the CMAS-resistant layer 15, along with any otherdesired additive components.

Other coating geometries may also be used to reduce the stress placed onthe interface of CMAS-resistant layer 15 and TBC/EBC 13 during thermalcycles due to different coefficients of thermal expansion. For example,as shown in FIG. 2, another article 20 may include a substrate 22 and acoating 24. The coating 24 may include a bond coat 21, a TBC/EBC 23, anda CMAS-resistant layer 25, as in FIG. 1. However, unlike the embodimentshown in FIG. 1, the coating 24 shown in FIG. 2 further includes atransitional layer 28 between the CMAS-resistant layer 25 and theTBC/EBC 23. The transitional layer 28 may include components of both theCMAS-resistant layer 25 and the TBC/EBC 23. For example, when a TBC/EBC23 includes a TBC comprising zirconia and the CMAS-resistant layer 25includes ytterbium silicate, the transitional layer 28 may include amixture of zirconia and ytterbium silicate. The mixture may include anapproximately equal amount of the components of the CMAS-resistant layer25 and TBC/EBC 23, or may include any other desired mixture orproportion of components from the CMAS-resistant layer 25 and TBC/EBC23.

The transitional layer 28 may be applied as a separate layer from theCMAS-resistant layer 25 and the TBC/EBC 23. For example, the TBC/EBC 23may be applied first by plasma spraying. The desired mixture of TBC/EBC23 components and CMAS-resistant layer 25 components may then be mixedand applied to the TBC/EBC 23 by plasma spraying, followed byapplication of pure CMAS-resistant layer 25 on the transitional layer28.

Additionally, as shown in FIG. 3, the transitional layer 38 may alsoinclude more than one sub-layer. In this embodiment, the transitionallayer 38 includes three sub-layers 38 a, 38 b, 38 c. However, thetransitional layer (e.g., transitional layer 38) may include as many oras few sub-layers as is desired. For example, transitional layer 38 mayinclude one layer, up to three layers, three layers, or more than threelayers.

Sub-layer 38 a is preferably compositionally most similar to TBC/EBC 33,e.g., sub-layer 38 a may include more than 50% (by weight, volume,moles, or the like) of components that form TBC/EBC 33. For example,sub-layer 38 a may include about 90% (by weight, volume, moles, or thelike) TBC/EBC 33 components, and about 10% (by weight, volume, moles, orthe like) CMAS-resistant layer 35 components. Sub-layer 38 b, then, mayinclude an approximately equal amount of components from TBC/EBC 33 andCMAS-resistant layer 35, or approximately 50% (by weight, volume, moles,or the like) TBC/EBC 33 components, and about 50% (by weight, volume,moles, or the like) CMAS-resistant layer 35 components. Finally,sub-layer 38 c may be more compositionally similar to the CMAS-resistantlayer. For example, sub-layer 38 c may include more than 50% (by weight,volume, moles, or the like) of CMAS-resistant layer 35 components. Inone embodiment, sub-layer 38 c may include about 90% (by weight, volume,moles, or the like) CMAS-resistant layer 35 components and about 10% (byweight, volume, moles, or the like) TBC/EBC 33 components.

The inclusion of the transitional layer 28, 38 may reduce thecoefficient of thermal expansion gradient, or in other words, make thecompositional transition from the TBC/EBC 23, 33 to the CMAS-resistantlayer 25, 35 more gradual, thus making the change of coefficients ofthermal expansion more gradual. FIGS. 4A, 4B, and 4C illustrate simpleexamples of the reduced forces exerted on the interface of adjacentlayers as the number of transitional layers is increased. In thefollowing examples, it is assumed that the CMAS-resistant layers have agreater coefficient of thermal expansion than the TBC/EBC layers. Whilein practice this may or may not be true, it is convenient for the sakeof these simple examples. Additionally, expansion is shown as occurringin only the horizontal direction of FIGS. 4A-C, which may or may not betrue in real systems. In real systems, the expansion may occur equallyin all directions, may occur in a greater amount in one or moredirection than in another direction, or may occur in inconsistentamounts throughout the material, depending on the temperature profile inthe material and the isotropy or anisotropy of the material, forexample.

As one example, FIG. 4A shows a CMAS-resistant layer 451 locatedimmediately adjacent TBC/EBC 431. Upon heating (indicated by arrow 401),the CMAS-resistant layer 451 expands laterally further than TBC/EBC 431expands. Line 491 in FIG. 4A indicates a vertical slice of the TBC/EBC431 and CMAS-resistant layer 451. As the thermal expansion progresses,line 491 symbolically breaks into two sections 491 a and 491 bcorresponding to CMAS-resistant layer 451 and TBC/EBC 431, respectively.Additionally, this expansion is indicated by gap 4 a, the distancebetween line sections 491 a and 491 b. The difference in rates or extentof expansion, then, causes a stress at the interface of TBC/EBC 431 andCMAS-resistant layer 451.

FIG. 4B, then, includes a transitional layer 482 between theCMAS-resistant layer 452 and TBC/EBC 432. As the article is heated(indicated by arrow 402), the transitional layer 492 expands anintermediate amount, more than the TBC/EBC 432 but less thanCMAS-resistant layer 452. This reduces the strain exerted on interface408 between the TBC/EBC 432 and the transitional layer 482 and oninterface 409 between the transitional layer 482 and CMAS-resistantlayer 452. As indicated by gap 4 b, the difference in expansion of thetransitional layer 482 and the CMAS-resistant layer 452, also shown asthe distance between line 492 a and line 492 b, is lower in FIG. 4B thanin 4A, while the expansion of the CMAS-resistant layers 452 and 451 andTBC/EBCs 431 and 432, respectively, are the same.

As a final example, FIG. 4C shows a transitional layer 483 that iscompositionally graded. That is, each of sub-layers 483 a, 483 b, 483 cis a different composition, with sub-layer 483 a being compositionallymost similar to TBC/EBC 133, sub-layer 483 c being compositionally mostsimilar to CMAS-resistant layer 453, and sub-layer 483 b beingcompositionally intermediate. This construction further reduces thestrain on the article due to thermal expansion caused by heat(represented by arrow 402). The difference in thermal expansion betweeneach adjacent layer, which is represented by gap 4 c, is smaller thaneither of gaps 4 a or 4 b, which indicated lower thermal expansiondifferences, and thus lower stresses on the interfaces between thelayers.

It may be understood that the more sub-layers included in thetransitional layer, the lower the interfacial stresses due to mismatchesof coefficients of thermal expansion. The number of sub-layers in thetransitional layer need not be limited, but may be chosen according tothe desired properties of the article and the time and expense involvedin producing the article.

The article may also include a transition layer that is not divided intosub-layers, but which includes a continuously graded composition. Forexample, the transition layer may be compositionally most similar to theTBC/EBC at the TBC/EBC-transitional layer interface, and most similar tothe CMAS-resistant layer at the CMAS-resistant layer-transitional layerinterface, with a composition that continuously transitions from theTBC/EBC composition to the CMAS-resistant layer composition along thedepth of the transitional layer.

As shown in FIG. 5, the CMAS-resistant layer 55 may also replace the TBCor EBC in some embodiments. Replacing the TBC or EBC with aCMAS-resistant layer 55 may allow better matching of the properties ofthe substrate 52 and the CMAS-resistant layer 55 than substrate 52 and aTBC or EBC (e.g., the coefficient of thermal expansion). TheCMAS-resistant layer 55 may provide one or more of the above-describedbenefits, including, for example, CMAS resistance, coefficient ofthermal expansion matching, and the like, while still providing lowthermal conductivity similar to or better than conventional TBCs.

The article 50 may include a CMAS-resistant layer 55 applied to a bondcoat 51, as shown in FIG. 5, or the CMAS-resistant layer 55 may beapplied directly to the substrate 52. The CMAS-resistant layer may againinclude at least one rare earth oxide. Useful rare earth oxides includeoxides of rare earth elements, including, for example, Sc, Y, La, Ce,Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinationsthereof. In some embodiments, the CMAS-resistant layer 55 may beessentially free of zirconia and hafnia. That is, in these embodiments,the coating includes at most trace amounts of zirconia and hafnia, suchas, for example, the amounts present in commercially-available rareearth oxides.

When CMAS-resistant layer 55 replaces the TBC or EBC, the CMAS-resistantlayer 55 may also include alumina, silica, or a mixture of alumina andsilica.

In some embodiments, the CMAS-resistant layer 55 may optionally includeup to about 50 mol. % additive components, such as, for example, Ta₂O₅,HfSiO₄, alkali oxides, alkali earth oxide, or mixtures thereof. Theadditive components may be added to the CMAS-resistant layer 55 tomodify one or more desired properties. For example, the additivecomponents may increase or decrease the reaction rate of theCMAS-resistant layer 55 with CMAS, may modify the viscosity of thereaction product from the reaction of CMAS and the CMAS-resistant layer55, may increase adhesion of the CMAS-resistant layer 55 to the bondcoat 51 or substrate 52, may increase or decrease the chemical stabilityof the CMAS-resistant layer 55, or the like.

In some preferred embodiments, the CMAS-resistant layer 55 may includeabout 10 mol. % to about 90 mol. % of at least one rare earth oxide andabout 10 mol. % to about 90 mol. % of at least one of alumina, silica,and combinations thereof, and optionally, about 0.1 mol. % to about 50mol. % of the additive components. In other preferred embodiments, theCMAS-resistant layer 15 may include about 20 mol. % to about 80 mol. %of at least one rare earth oxide and about 20 mol. % to about 80 mol. %of at least one of alumina, silica, and combinations thereof, andoptionally, about 1 mol. % to about 30 mol. % of the additivecomponents. All measurements are ±1 mol. %

The CMAS-resistant layer may be applied to the bond coat 51 or substrate52 using any useful technique, including, for example, electron beamphysical vapor deposition, plasma spraying, chemical vapor deposition,and the like.

EXAMPLES Example 1

FIGS. 6A and 6B show two cross-sectional photographs of an article 60including an ytterbium silicate (Yb₂Si₂O₇ or Yb₂O₃-2SiO₂) layer 62adjacent a CMAS layer 64. The article 60 has been exposed to atemperature of 1250° C. for 4 hours prior to the photograph. The CMASand ytterbia silicate have reacted at the interface to form a thin(about 5 μm thick) reaction layer 66. A portion 68 of the article 60,which shows the reaction layer 66 more clearly, is shown in FIG. 6B.

Example 2

FIGS. 7A and 7B show two cross-sectional photographs of an article 70including an ytterbium silicate (Yb₂Si₂O₇ or Yb₂O₃-2SiO₂) layer 72adjacent a CMAS layer 74. The article 70 has been exposed to atemperature of 1350° C. for 4 hours prior to the photograph. The CMASand ytterbium silicate have reacted at the interface to form a reactionlayer 76. A portion 78 of the article 70 is shown in FIG. 7B.

Example 3

FIGS. 8A and 8B show two cross-sectional photographs of an article 80after exposure to a temperature of 1350° C. for 4 hours. The articleincludes a layer 82 of a second type of ytterbium silicate (Yb₂SiO₅ orYb₂O₃—SiO₂) adjacent a CMAS layer 84. The CMAS and ytterbia silicatehave again reacted at the interface and formed a reaction layer 86 thatis about 10 μm thick. Additionally, FIG. 8A shows cracks 88, 89 in theytterbium silicate layer 82. CMAS has not infiltrated the cracks 88, 89,which indicate that the reaction layer 86 quickly formed an effectivebarrier to molten CMAS. A portion 88 of the article 80, which shows thereaction layer 86 more clearly, is shown in FIG. 8B.

Example 4

FIG. 9 shows an ytterbium silicate (Yb₂SiO₅ or Yb₂O₃—SiO₂) layer 92contacting CMAS layers 94, 95 on both surfaces of the layer 92 afterexposure to a temperature of 1450° C. for 4 hours. Reaction layers 96,97 have formed on both surfaces of the ytterbium silicate layer 92.

Example 5

FIG. 10 shows an article 100 including a CMC substrate 101, a siliconbond coat 103 applied to substrate 101, and an ytterbium silicate(Yb₂Si₂O₇ or Yb₂O₃-2SiO₂) layer 102 applied to the bond coat 103. Thearticle was exposed to one hundred 1 hour thermal cycles at 1300° C.(2372° F.) and 90% water vapor in oxygen, which mimics the conditions ofa combustion section of a gas turbine engine. After the thermal cycling,the ytterbium silicate layer remains well-adhered to the CMC substrate101, and the substrate 101 shows no evidence of damage.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1-44. (canceled)
 45. A coating comprising a CMAS-resistant layercomprising a rare earth oxide and alumina, wherein the CMAS-resistantlayer is essentially free of zirconia and hafnia.
 46. The coating ofclaim 45, wherein the CMAS-resistant layer further comprises silica. 47.The coating of claim 46, wherein the CMAS-resistant layer comprises 1mol. % to 100 mol. % of at least one rare earth oxide and up to 99 mol.% of at least one of alumina, silica and combinations thereof, with atotal of 100 mol. %.
 48. The coating of claim 47, wherein theCMAS-resistant layer further comprises 0.1 mol. % to 50 mol. % of anadditive component selected from Ta₂O₅, HfSiO₄, alkali oxides, alkaliearth oxides, and combinations thereof.
 49. The coating of claim 46,wherein the CMAS-resistant layer comprises 10 mol. % to 90 mol. % of atleast one rare earth oxide, and 10 mol. % to 90 mol. % of at least oneof alumina, silica and combinations thereof, with a total of 100 mol. %.50. The coating of claim 49, wherein the CMAS-resistant layer furthercomprises 0.1 mol. % to 50 mol. % of an additive component selected fromTa₂O₅, HfSiO₄, alkali oxides, alkali earth oxides, and combinationsthereof.
 51. The coating of claim 46, wherein the CMAS-resistant layercomprises 20 mol. % to 80 mol. % of at least one rare earth oxide and 20mol. % to 80 mol. % of at least one of alumina, silica and combinationsthereof, with a total of 100 mol. %.
 52. The coating of claim 51,wherein the CMAS-resistant layer further comprises 1 mol. % to 30 mol. %of an additive component selected from Ta₂O₅, HfSiO₄, alkali oxides,alkali earth oxides, and combinations thereof.
 53. The coating of claim45, wherein the rare earth oxide is selected from the group consistingof oxides of scandium, yttrium, lanthanum, cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, lutetium, andcombinations thereof.
 54. The coating of claim 45, wherein theCMAS-resistant layer is about 0.1 mil to about 60 mil thick.
 55. Thecoating of claim 45, wherein the CMAS-resistant layer is about 0.5 milto about 15 mil thick.
 56. The coating of claim 45, wherein the coatingfurther comprises a second layer comprising at least one of: a MCrAlYalloy, wherein M is selected from Ni, Co, and NiCo; a β-NiAl alloy; aγ-Ni+γ′-Ni₃Al alloy; rare earth oxide-stabilized zirconia, rare earthoxide-stabilized hafnia, mullite, silicon, barium strontiumaluminosilicate, calcium aluminosilicate, cordierite, lithiumaluminosilicate, rare earth silicates, and combinations thereof, whereinthe CMAS-resistant layer is adjacent the second layer.
 57. The coatingof claim 56, wherein the β-NiAl alloy further comprises at least one ofPt, Cr, Hf, Zr, Y, Si, and combinations thereof.
 58. The coating ofclaim 56, wherein the γ-Ni+γ′-Ni₃Al alloy further comprises at least oneof Pt, Cr, Hf, Zr, Y, Si, and combinations thereof.
 59. The coating ofclaim 56, further comprising a transitional layer, wherein thetransitional layer is adjacent the second layer and wherein theCMAS-resistant layer is adjacent the transitional layer.
 60. A coatingcomprising: a CMAS-resistant layer comprising a rare earth oxide; and anenvironmental barrier coating (EBC), wherein the CMAS-resistant layer isadjacent the EBC.
 61. The coating of claim 60, wherein theCMAS-resistant layer further comprises at least one of alumina, silica,and combinations thereof.
 62. The coating of claim 60, wherein the rareearth oxide is selected from scandia, yttria, lanthana, ceria,praseodymia, neodymia, promethia, samaria, europia, gadolinia, terbia,dysprosia, holmia, erbia, thulia, ytterbia, lutetia, and combinationsthereof.
 63. The coating of claim 60, wherein the CMAS-resistant layeris about 0.1 mil to about 60 mil thick.
 64. The coating of claim 60,further comprising a transitional layer, wherein the transitional layeris adjacent the EBC and wherein the CMAS-resistant layer is adjacent thetransitional layer.
 65. The coating of claim 61, wherein theCMAS-resistant layer comprises 1 mol. % to 100 mol. % of at least onerare earth oxide and up to 99 mol. % of at least one of alumina, silicaand combinations thereof, with a total of 100 mol. %.
 66. The coating ofclaim 65, wherein the CMAS-resistant layer further comprises 0.1 mol. %to 50 mol. % of an additive component selected from Ta₂O₅, HfSiO₄,alkali oxides, alkali earth oxides, and combinations thereof.
 67. Thecoating of claim 61, wherein the CMAS-resistant layer comprises 10 mol.% to 90 mol. % of at least one rare earth oxide, and 10 mol. % to 90mol. % of at least one of alumina, silica and combinations thereof, witha total of 100 mol. %.
 68. The coating of claim 67, wherein theCMAS-resistant layer further comprises 0.1 mol. % to 50 mol. % of anadditive component selected from Ta₂O₅, HfSiO₄, alkali oxides, alkaliearth oxides, and combinations thereof.
 69. The coating of claim 61,wherein the CMAS-resistant layer comprises 20 mol. % to 80 mol. % of atleast one rare earth oxide and 20 mol. % to 80 mol. % of at least one ofalumina, silica and combinations thereof, with a total of 100 mol. %.70. The coating of claim 69, wherein the CMAS-resistant layer furthercomprises 1 mol. % to 30 mol. % of an additive component selected fromTa₂O₅, HfSiO₄, alkali oxides, alkali earth oxides, and combinationsthereof.
 71. An article comprising: a substrate; and a CMAS-resistantlayer comprising a rare earth oxide and alumina, wherein theCMAS-resistant layer is essentially free of zirconia and hafnia.
 72. Thearticle of claim 71, wherein the CMAS-resistant layer further comprisessilica.
 73. The article of claim 72, wherein the CMAS-resistant layercomprises 1 mol. % to 100 mol. % of at least one rare earth oxide and upto 99 mol. % of at least one of alumina, silica and combinationsthereof, with a total of 100 mol. %.
 74. The article of claim 73,wherein the CMAS-resistant layer further comprises 0.1 mol. % to 50 mol.% of an additive component selected from Ta₂O₅, HfSiO₄, alkali oxides,alkali earth oxides, and combinations thereof.
 75. The article of claim71, wherein the rare earth oxide is selected from scandia, yttria,lanthana, ceria, praseodymia, neodymia, promethia, samaria, europia,gadolinia, terbia, dysprosia, holmia, erbia, thulia, ytterbia, lutetia,and combinations thereof.
 76. The article of claim 71, wherein theCMAS-resistant layer overlies at least a portion of the substrate,wherein the CMAS-resistant layer comprises a first surface adjacent thesubstrate and a second surface opposite the substrate, and wherein theCMAS-resistant layer reacts with CMAS to form a reaction product layeron the second surface of the CMAS-resistant layer.
 77. The article ofclaim 71, wherein the article further comprises a bond coat, and whereinthe bond coat is adjacent the substrate and the CMAS-resistant layer isadjacent the bond coat.
 78. The article of claim 71, wherein the articlefurther comprises an environmental barrier coating (EBC), and whereinthe EBC is adjacent the substrate and the CMAS-resistant layer isadjacent the EBC.
 79. The article of claim 71, wherein the articlefurther comprises an environmental barrier coating (EBC) and a bondcoat, and wherein the bond coat is adjacent the substrate, the EBC isadjacent the bond coat, and the CMAS-resistant layer is adjacent theEBC.
 80. The article of claim 79, wherein the article further comprisesa transitional layer, wherein the transitional layer is adjacent the EBCand the CMAS-resistant layer is adjacent the transitional layer, andwherein the transitional layer comprises a mixture of components of theEBC and components of the CMAS-resistant layer.
 81. The article of claim71, wherein the article further comprises a thermal barrier coating(TBC), and wherein the TBC is adjacent the substrate and theCMAS-resistant layer is adjacent the TBC.
 82. The article of claim 81,wherein the article further comprises a transitional layer wherein thetransitional layer is adjacent the TBC and the CMAS-resistant layer isadjacent the transitional layer, and wherein the transitional layercomprises a mixture of components of the TBC and components of theCMAS-resistant layer.
 83. The article of claim 71, wherein the articlefurther comprises a thermal barrier coating (TBC) and a bond coat, andwherein the bond coat is adjacent the substrate, the TBC is adjacent thebond coat, and the CMAS-resistant layer is adjacent the TBC.
 84. Thearticle of claim 83, wherein the article further comprises atransitional layer, wherein the transitional layer is adjacent the TBCand the CMAS-resistant layer is adjacent the transitional layer, andwherein the transitional layer comprises a mixture of components of theTBC and components of the CMAS-resistant layer.
 85. An articlecomprising: a substrate; a bond coat on the substrate; and aCMAS-resistant layer on the bond coat, wherein the CMAS-resistant layercomprises a rare earth oxide and is essentially free of zirconia andhafnia.
 86. The article of claim 85, wherein the CMAS-resistant layerfurther comprises at least one of alumina, silica, and combinationsthereof.
 87. The article of claim 86, wherein the CMAS-resistant layercomprises 1 mol. % to 100 mol. % of at least one rare earth oxide and upto 99 mol. % of at least one of alumina, silica and combinationsthereof, with a total of 100 mol. %.
 88. The article of claim 87,wherein the CMAS-resistant layer further comprises 0.1 mol. % to 50 mol.% of an additive component selected from Ta₂O₅, HfSiO₄, alkali oxides,alkali earth oxides, and combinations thereof.
 89. The article of claim86, wherein the CMAS-resistant layer comprises 10 mol. % to 90 mol. % ofat least one rare earth oxide, and 10 mol. % to 90 mol. % of at leastone of alumina, silica and combinations thereof, with a total of 100mol. %.
 90. The article of claim 89, wherein the CMAS-resistant layerfurther comprises 0.1 mol. % to 50 mol. % of an additive componentselected from Ta₂O₅, HfSiO₄, alkali oxides, alkali earth oxides, andcombinations thereof.
 91. The article of claim 86, wherein theCMAS-resistant layer comprises 20 mol. % to 80 mol. % of at least onerare earth oxide and 20 mol. % to 80 mol. % of at least one of alumina,silica and combinations thereof, with a total of 100 mol. %.
 92. Thearticle of claim 91, wherein the CMAS-resistant layer further comprises1 mol. % to 30 mol. % of an additive component selected from Ta₂O₅,HfSiO₄, alkali oxides, alkali earth oxides, and combinations thereof.93. The article of claim 85, wherein the rare earth oxide is selectedfrom the group consisting of oxides of scandium, yttrium, lanthanum,cerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,lutetium, and combinations thereof.
 94. The article of claim 85, whereinthe bond coat comprises at least one of: a MCrAlY alloy, wherein M isselected from Ni, Co, and NiCo; a β-NiAl alloy; a γ-Ni+γ′-Ni₃Al alloy;mullite, silicon, and combinations thereof.
 95. The article of claim 94,wherein the β-NiAl alloy further comprises at least one of Pt, Cr, Hf,Zr, Y, Si, and combinations thereof.
 96. The coating of claim 94,wherein the γ-Ni+γ′-Ni3Al alloy further comprises at least one of Pt,Cr, Hf, Zr, Y, Si, and combinations thereof.
 97. The article of claim85, wherein the substrate comprises at least one of a superalloy or aceramic matrix composite (CMC).
 98. An article comprising: a substratecomprising a ceramic matrix composite or a ceramic; and a coatingcomprising a CMAS-resistant layer comprising a rare earth oxide, whereinthe CMAS-resistant layer is essentially free of zirconia and hafnia. 99.The article of claim 98, wherein the CMAS-resistant layer furthercomprises at least one of silica or alumina.
 100. The article of claim99, wherein the CMAS-resistant layer comprises silica, and wherein atleast some of the silica and at least some of the rare earth oxide format least one of a rare earth monosilicate or a rare earth disilicate.101. The article of claim 99, wherein the CMAS-resistant layer comprises1 mol. % to 100 mol. % of at least one rare earth oxide and up to 99mol. % of at least one of alumina or silica.
 102. The article of claim101, wherein the CMAS-resistant layer further comprises 0.1 mol. % to 50mol. % of an additive component selected from Ta₂O₅, HfSiO₄, alkalioxides, alkali earth oxides, and combinations thereof.
 103. The articleof claim 99, wherein the CMAS-resistant layer comprises 10 mol. % to 90mol. % of at least one rare earth oxide and 10 mol. % to 90 mol. % of atleast one of alumina or silica.
 104. The article of claim 103, whereinthe CMAS-resistant layer comprises 0.1 mol. % to 50 mol. % of anadditive component selected from Ta₂O₅, HfSiO₄, alkali oxides, alkaliearth oxides, and combinations thereof.
 105. The article of claim 99,wherein the CMAS-resistant layer comprises 20 mol. % to 80 mol. % of atleast one rare earth oxide and 20 mol. % to 80 mol. % of at least one ofalumina or silica.
 106. The article of claim 105, wherein theCMAS-resistant layer further comprises 1 mol. % to 30 mol. % of anadditive component selected from Ta₂O₅, HfSiO₄, alkali oxides, alkaliearth oxides, and combinations thereof.
 107. The article of claim 98,wherein the rare earth oxide is selected from the group consisting ofoxides of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, lutetium, and combinations thereof.108. The article of claim 98, wherein the CMAS-resistant layer comprisesa thickness of about 0.1 mil to about 60 mil.
 109. The article of claim98, wherein the CMAS-resistant layer comprises a thickness of about 0.5mil to about 15 mil.
 110. The article of claim 98, wherein the coatingfurther comprises a second layer comprising at least one of mullite,silicon, barium strontium aluminosilicate, calcium aluminosilicate,cordierite, lithium aluminosilicate, or a rare earth silicate, whereinthe CMAS-resistant layer is adjacent to the second layer.
 111. Thearticle of claim 110, wherein the second layer comprises silicon. 112.The article of claim 111, wherein the CMAS-resistant layer comprises arare earth silicate.
 113. The article of claim 110, further comprising atransitional layer, wherein the transitional layer is adjacent to thesecond layer and wherein the CMAS-resistant layer is adjacent to thetransitional layer.